It is often necessary, or at least desirable, to concentrate an aqueous liquid, such as a solution or mixture, by removing a portion of the solvent, generally water, from the aqueous liquid. The resulting product, therefore, is in a more concentrated form. Some such products which are concentrated by removal of water are fruit and vegetable juices including orange juice, grapefruit juice, lime juice, apple juice and grape juice; vegetable juices such as pineapple juice, tomato juice and cranberry juice; wine, alcoholic beverages such as beer and ale and non-alcoholic beverages such as coffee and tea.
Because of the shortcomings involved in evaporative concentration, it has been found advantageous to freeze concentrate many aqueous liquids, or products, such as those just described. In such a process, water is removed by first producing ice crystals which are then separated from the concentrate. Next, the ice crystals are washed to remove the concentrate remaining on them. The ice crystals can then be discarded or melted if potable water is desired.
It is recognized that the viscosity of products being freeze concentrated may substantially increase as water is removed, especially products containing dissolved sugars and suspended solids. Processing of products with increased viscosity requires substantial energy consumption since they are difficult to pump, and gravity flow is relatively slow. Also, it is difficult to recover a viscous concentrate from ice crystals by washing. When concentrating a product such as orange juice, even a small loss of entrained concentrate is quite detrimental economically. See C. Judson King, "Separation Processes", McGraw-Hill, page 725. Removal of the entrained concentrate from ice crystals by washing would be most effective with a low product viscosity and large ice crystals.
Abraham Ogman's U.S. Pat. No. 4,091,635 discloses an apparatus form and method of, freeze concentrating an already concentrated feed stream. Ogman employs a two stage system in which each stage uses a freezer-crystallizer and a washer. In the first stage, the concentration is doubled. Ice from the first stage is then brought to the second stage and diluted with a feed stream of low concentration following which the diluted solution is freeze concentrated in the second stage.
A two stage process has three important drawbacks when compared to a three stage process. First, final washing of ice crystals takes place at a relatively high viscosity, which results in inadequate washing of ice crystals. Second, most of the ice crystals may have to be produced at a high viscosity resulting in smaller ice crystals. Third, the energy consumption is very high.
Gerald E. Engdahl's U.S. Pat. No. 4,457,769 also discloses an apparatus for, and method of, freeze concentration. Engdahl is directed primarily to use of a crystallizer in which ice crystals can grow to thereby provide a slurry from which the liquid concentrate can be more readily separated and washed. It does, however, disclose use of two freeze exchangers in series and a conventional washer to separate concentrate from ice. While the Engdahl system constitutes an improvement over earlier systems, it requires more energy consumption to operate than desired. In addition, it involves problems in handling the more viscous concentrates which are desired commercially so that less water content need be shipped with the product concentrate.
Another drawback is that in such a system which is not totally enclosed, there is a large interface between the aqueous liquid and the surrounding atmosphere in each stage making it difficult to have an inert gas blanket over the aqueous liquid. This can affect the quality of the concentrate.
For the above reasons, a need exists for alternative apparatus and methods for concentrating aqueous liquids such as those previously described, which increase in viscosity with increase in concentration and where it is necessary to preserve highly volatile flavors and aromas in the concentrate.